There is literally no source of value other than labor.
While Marx believed this, it's not true. The source of value is "human want". If humans don't want your labour, it has no value.
It can be true, in a sense, but
only in a market-driven system.
The labor theory of value (LTV) is a theory of value that argues that the economic value of a good or service is determined by the total amount of "socially necessary labor" required to produce it.
Breaking this down. If some particular person
can make 10 of something in an hour, then the socially
necessary labor to produce one unit is 6 minutes. So, if a minute is a dollar then the value is 6 "minute-dollars". Nothing in this guarantees that everyone's labor is equally valuable. If you can only make 5 per hour, your labor is only worth half as a much: the value of what you made is half the economic value of what someone else made.
Where the market stipulation comes in, is that the "socially necessary labor" determines per-unit costs, but total price is constrained by supply and demand considerations. Unless you get cartels, monopolies etc. Take the example of someone selling bottled water in the desert. It could be argued that the value of the water is the scarcity, but actually it's the fact that firstly, they have a monopoly in that scenario, and secondly, the effort for someone to actually go and get more water themselves is quite high, thus they're willing to pay you for the water you already got.